They will be allowed to patrol the streets fully armed wherever Queen Elizabeth goes on her four-day visit, which has sparked the biggest security operation in the history of the State.

While security sources in Dublin and London played down reports that Republican terrorists had been trying to buy rocket-launchers ahead of the visit, the operation to prevent an attack was this weekend starting to crank up.

Roads around the capital were closed off yesterday. Within days, armed gardaí will be joined by British officers.

While personal bodyguards of visiting dignitaries are usually allowed to carry firearms, the scale of the Met force will prove controversial on what is fast becoming a contentious visit.

Yesterday, Sinn Féin president Gerry Adams was forced to deny reports that he was welcoming the Queen, and singer Mary Byrne and traditional group the Chieftains have endured savage online attacks for daring to play at a concert in her honour.

However, the Chieftains' Paddy Moloney has defended the band's decision; and writing in today's Irish Mail on Sunday, Éamon de Valera's grandson Éamon Ó Cuív said he too was entirely supportive.

A senior Government source told the MoS that the decision to admit 120 armed Met officers had been an easy one to make.

They aren't just landing without warning  the Brits had to request permission from the Irish authorities to allow this, and it was granted without hesitation,' he said.

'This is the norm. When our President goes abroad she has to ask if armed gardaí can escort her.

'It is certainly unusual to see police carrying guns in Ireland, but it happens.

'When Obama comes, there will be triple that amount of armed U.S. cops.'

Nevertheless, ministers recognise the difference in political terms between the U.S. Secret Service taking to Irish streets and representatives of the British police arriving here in force.

However, senior Government officials said they were hoping that the security precautions would minimise the necessity of any officers having to act.

'The Queen will have armed protection officers, but we are not anticipating any situation where troublemakers can get near her.

'There are no events where unscreened members of the public can get near to Queen Elizabeth,' said a senior Government official.

'Of course the public are free to line the route she takes from the Phoenix Park to the Garden of Remembrance or on Patrick Street in Cork.'

One Republican group, Éirigí, is expected to stage a protest when the Queen lays a wreath at the Garden of Remembrance on Parnell Square.

Around 250 police and gardaí will shadow the Queen when she visits Dublin, Cork and Tipperary.

The source said the gardaí have 'no fear' of any security breach during the visit.

'Security is so tight. There will be a few a******s out to cause trouble and something will kick off, but nothing major.'

Wherever she goes, barriers will ensure no one gets near Queen Elizabeth, the source said.

'Barricades will be 40 feet away. She will never be close enough for anyone to throw anything at her or touch her.'